The automotive industry is increasingly using ultra-high strength material sheets to increase structural strength and deliver increased safety, or to reduce weight and deliver improved fuel economy, or both. Ultra-high strength steels currently typically have a tensile strength from about 800 MPa to about 1200 MPa. Research is ongoing, however, to develop and produce ultra-high strength steels with tensile strengths of 1500 MPa, 1800 MPa, and even higher. The higher the tensile strength of a material, the more difficulties and problems occur with cutting the material.
When attempting to cut or shear an interior opening in an ultra-high strength material sheet using a traditional process, the cutting tool must initially break through the material. Such a breakthrough operation requires generating a very high peak blanking or cutting force. At the moment of breakthrough, there is a significant, virtually instantaneous drop in the blanking or cutting force. Thus, a loud noise, perhaps at 105 dB or greater, accompanies the moment of breakthrough. In addition, a significant vibration passes through the building, feeling somewhat like an earthquake. Such loud sounds and large vibrations can be very problematic to workers, machines, as well as tools and dies, in manufacturing facilities; particularly when there are fast cycle times. Just as problematic is the wear and tear on the manufacturing machines that are repeatedly subjected to these significant, virtually instantaneous drops in the blanking or cutting force.
Another disadvantage relates to the leading edge of the cutting tool that is engaged against the ultra-high strength sheet during the initial breakthrough operation. Such tools typically involve two angled surfaces coming together at a point, similar to a roof or V-shaped configuration used for balancing induced horizontal surfaces. Thus, such tools typically define two cutting or shearing edges on opposite sides of the angled surfaces that each has a V-shaped configuration with the apex of the V-shape providing the initial or leading contact point or edge along with a pointed roof line extending between them. When cutting ultra-high strength material sheets, these tools wear prematurely at the leading contact points or leading terminal ends. For example, the three-sided corners at the apex of each V-shaped side are often subject to balling, chipping or cracking, long before the useful life of the remainder of the shearing edges have worn down. This premature failure of these tools is problematic because shearing tools capable of cutting ultra-high strength material sheets are expensive to manufacture and purchase.
The present disclosure provides a two-stage method of cutting an interior opening in ultra-high strength material sheets, such as ultra-high strength steel sheets, that solves both the highly localized premature tool failure issues and the operational issues associated with high breakthrough forces.